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Japanese Journal of Applied Physics 57, 020303 (2018)https://doi.org/10.7567/JJAP.57.020303

Detection of 2-mm-long strained section in silica ﬁber using slope-assisted
Brillouin optical correlation-domain reﬂectometry
Heeyoung Lee*, Yosuke Mizuno, and Kentaro Nakamura
Institute of Innovative Research, Tokyo Institute of Technology, Yokohama 226-8503, Japan*E-mail: hylee@sonic.pi.titech.ac.jp
Received October 25, 2017; accepted November 28, 2017; published online December 27, 2017
Slope-assisted Brillouin optical correlation-domain reﬂectometry is a single-end-access distributed Brillouin sensing technique with high spatial
resolution and high-speed operation. We have recently discovered its unique feature, that is, strained or heated sections even shorter than nominal
resolution can be detected, but its detailed characterization has not been carried out. Here, after experimentally characterizing this “beyondnominal-resolution” effect, we show its usefulness by demonstrating the detection of a 2-mm-long strained section along a silica ﬁber. We also
demonstrate the detection of a 5-mm-long heated section along a polymer optical ﬁber. The lengths of these detected sections are smaller thanthose of the other demonstrations reported so far. © 2018 The Japan Society of Applied Physics

ptical ﬁber sensing based on Brillouin scattering1)
can provide information on the distributions ofstrain and temperature change, and is regarded
as a promising technique for structural health monitoring.
These days, with enhanced spatial resolution, not only civil
infrastructures but also much smaller structures, such as
optical waveguides, are focused on as sensing targets. Hence,
many types of Brillouin-based measurement schemes havebeen developed, such as Brillouin optical time-domain
reﬂectometry (BOTDR),2–5) Brillouin optical frequencydomain reﬂectometry (BOFDR),6) Brillouin optical correlation-domain reﬂectometry (BOCDR),7–10) Brillouin optical
time-domain analysis (BOTDA),11–15) Brillouin optical frequency-domain analysis (BOFDA),16–18) and Brillouin optical correlation-domain analysis (BOCDA).19–23) While eachscheme has its own merit and demerit, BOCDR is the only
technique that operates by inherently one-end light injection
and has high spatial resolution and cost eﬃciency, although it
suﬀers from the trade-oﬀ relationship between the spatial
resolution and the measurement range.
A number of BOCDR conﬁgurations have been implemented to improve the performance24–27) since the ﬁrst development of its original conﬁguration.7) As for the operating
speed, the sampling rate of BOCDR has been limited to 19
Hz,8) resulting in the diﬃculty in real-time distributed measurement. In 2016, however, we developed two high-speed
conﬁgurations of BOCDR: phase-detected BOCDR28) andslope-assisted (SA-) BOCDR.29) In phase-detected BOCDR,
the Brillouin gain spectrum (BGS) is converted to timedomain signal, and it is approximated by a sinusoidal waveform, the phase delay of which is directly obtained. By thistechnique, a 100 kHz sampling rate was achieved, and realtime distributed strain sensing was demonstrated. However,the limited strain dynamic range was a major problem,
although some eﬀorts to mitigate this limitation have been
conducted.30) Thus, here we focus on SA-BOCDR.
In SA-BOCDR, the spectral power of the BGS is used toobtain the Brillouin frequency shift (BFS) by exploiting their
one-to-one correspondence. The basic operations of SABOCDR have been demonstrated using not only silica glass
optical ﬁbers29) but also polymer optical ﬁbers (POFs).31)
When POFs were used, 100-mm-long strained and heatedsections were successfully detected.32) Recently, the relationship between the ﬁnal system output (i.e., power-change distribution) and the actual BFS distribution has been inves-O

Fig. 1. (Color online) Experimental setup for SA-BOCDR. EDFA:
erbium-doped ﬁber ampliﬁer, ESA: electrical spectrum analyzer, FUT: ﬁber
under test, OSC: oscilloscope, PD: photo diode, PSCR: polarization
scrambler.

tigated.33) In addition, it has been proved that, even when thestrained (or heated) section is shorter than the nominal spatial
resolution, some shift in the power change can be observed.33)
Although this “beyond-nominal-resolution” eﬀect unique to
SA-BOCDR is very attractive for practical applications, nodetailed characterization has been performed yet.
In this work, ﬁrst, we experimentally characterize thebeyond-nominal-resolution eﬀect of SA-BOCDR using asilica single-mode ﬁber (SMF). We ﬁnd that a strained sectionthat is over 50 times shorter than the nominal spatial resolution can be detected. Then, we show the usefulness of thebeyond-nominal-resolution eﬀect by detecting a 2-mm-longstrained section along a silica SMF and a 5-mm-long heatedsection along a POF. These values are smaller than the world
records previously reported, which are 334) and 100 mm32) fora silica SMF and a POF, respectively [both values wererestricted by the signal-to-noise ratios (SNRs) of the systems].
Figure 1 shows the experimental setup of SA-BOCDR
used in the experiment. It is basically the same as that reported
in our literature,29) to which reference should be made fora detailed explanation. A laser diode at 1550 nm with abandwidth of ∼1 MHz was used as a light source. Throughout
the experiment in this work, the video and resolution bandwidths of the electrical spectrum analyzer (ESA) were set to
3 kHz and 10 MHz, respectively, while the repetition rate was
100 Hz. Note that a polarization scrambler was inserted in thepump path in this setup, but it could be inserted immediately
after the laser output before an optical coupler.
First, to characterize the beyond-nominal-resolution eﬀect,
we employed a 14.0-m-long silica SMF as a ﬁber under test
(FUT). The output light from a laser was ampliﬁed to ∼25020303-1© 2018 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 57, 020303 (2018)

H. Lee et al.

(a)

(b)

(c)
(a)

Fig. 2. (Color online) (a) Structure of a silica SMF under test. The lengthof the strained section Lε was set to 0.01, 0.02, 0.05, 0.10, 0.20, 0.50, and
1.00 m. (b) Structure of a silica SMF under test, which was locally strainedfor 0, 0.10, 0.15, and 0.20%. (c) Structure of a POF under test, which was
locally heated to 26, 35, 45, and 55 °C.

dBm using an erbium-doped ﬁber ampliﬁer (EDFA) andinjected into the FUT. The BFS of the FUT was 10.89 GHz at
1.55 µm at 26 °C (room temperature). The open end of the
FUT was bent to suppress the Fresnel reﬂection. The spectral
power change at 10.87 GHz (determined by diﬀerentiating the
BGS29)) was monitored using an oscilloscope (OSC). The
modulation amplitude Δf and the modulation frequency fm
were set to 0.19 GHz and 5.10–5.26 MHz, respectively, leading to the nominal spatial resolution of 1.01 m and the measurement range of 20.1 m, calculated using Eqs. (15) and (16)in Ref. 8. A strain of 0.1% was applied to 0.01-, 0.02-, 0.05-,
0.10-, 0.20-, 0.50-, and 1.00-m-long sections of the FUT, as
shown in Fig. 2(a). When the length of the strained section
was changed, the position of its proximal end was ﬁxed.
Averaging was performed 512 times on the OSC to obtain ahigher SNR. The number of sampling points was 2500.
Subsequently, exploiting the beyond-nominal-resolution
eﬀect, we attempted to detect the shortest-ever strained andheated sections along a silica SMF and a POF. In this experiment, we injected ∼28 dBm light to the FUTs to enhance themeasurement sensitivity,35) and averaging was performed
1024 times. The number of sampling points was 2500. To
begin with, as an FUT, we employed a 2.0-m-long silica
SMF, the BFS of which was 10.85 GHz at room temperature.
We applied strains of 0.10, 0.15, and 0.20% to a 2-mm-longsection (ﬁxed on translation stages using epoxy glue; ∼55
times shorter than the nominal spatial resolution; see below),
as depicted in Fig. 2(b). The nominal spatial resolution andthe measurement range were 0.11 and 7.64 m (modulation
amplitude Δf: 0.67 GHz; modulation frequency fm: 13.45–
13.54 MHz), respectively. The change in the spectral power
at 10.84 GHz was observed using the OSC. Then, a 2.0-mlong perﬂuorinated graded-index (PFGI-) POF36) (core diameter: 50 µm; propagation loss at 1.55 µm: 0.25 dB=m; BFS atroom temperature: 2.75 GHz) was used as an FUT. A 5-mmlong section was heated to 35, 45, and 55 °C [see Fig. 2(c)].
The modulation amplitude Δf and the modulation frequency(b)Fig. 3. (Color online) (a) Normalized power-change distributions
measured when the length of the strained section Lε was reduced from 1.00 to
0.01 m; each distribution was shifted by 0.5. The dotted lines indicate thetheoretical trends. (b) Maximal power changes plotted as a function of Lε.
The error bars are calculated as standard deviations of the signal ﬂuctuations
at nonstrained sections. The dotted line is a linear ﬁt. The inset shows themagniﬁed view at Lε shorter than 0.1 m.

fm were set to 0.69 GHz and 24.70–24.84 MHz, respectively,
resulting in the nominal spatial resolution of 0.21 m and themeasurement range of 4.5 m.8) Note that the length of theheated section was over 40 times smaller than the nominal
spatial resolution. The change in the spectral power at 2.78
GHz was monitored.
The power-change distributions measured when the lengthof the strained section Lε was reduced from 1.00 to 0.01 m areshown in Fig. 3(a). Note that the nominal spatial resolution
was 1.01 m throughout the measurement. The vertical axis
was normalized so that the maximal power change became 1
when Lε was 1.00 m. Each distribution was displayed with avertical interval of 0.5. When Lε was 1.00 m, the powerchange distribution showed a triangular shape, which isnatural considering the operating mechanism of SABOCDR.33) As Lε decreased, the maximal power change also
decreased. When Lε was 0.02 m, the power change was stillclearly detected, but it became almost comparable to thesignal ﬂuctuations (calculated standard deviation: 0.06) whenLε was 0.01 m. Thus, under this experimental condition, astrained section that is over 50 times shorter than the nominal
spatial resolution was shown to be detectable. Figure 3(b)020303-2© 2018 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 57, 020303 (2018)

H. Lee et al.

(a)

(a)

(b)

(b)

Fig. 4. (Color online) (a) Normalized power-change distributions along asilica SMF measured at four strains. Each distribution was shifted by 1.
(b) Maximal power changes plotted as a function of strain. The dotted line isa linear ﬁt.

shows the normalized maximal power change plotted as afunction of Lε. The error bars were standard deviations of thesignal ﬂuctuations at nonstrained sections. The maximal
power change decreased almost linearly with decreasing Lε,which agrees well with the theory.33) The observed length of
the nonzero-power-change section was almost equal to thesum of the strained length Lε and the nominal spatial resolution, which also agrees with the theory33) (quantitative evaluation is diﬃcult because of the low SNR when Lε is short).
Next, exploiting the beyond-nominal-resolution eﬀect
veriﬁed above, we attempted to detect the shortest-everstrained and heated sections along a silica SMF and a POF.
Figure 4(a) shows the normalized power-change distributions
measured when strains of 0.10, 0.15, and 0.20% were applied
to a 2-mm-long section of the silica SMF. The local powerchanges corresponding to the strain were observed at thecorrect position. The lengths of the power-changed sections
appeared to be much larger than 2 mm; in theory,33) theyshould be almost the same as the nominal resolution (0.11 m),
which moderately agrees with the measured results. The
maximal normalized power change of each distribution in
Fig. 4(a) was plotted as a function of applied strain in
Fig. 4(b). With increasing strain, the power-change also
increased almost linearly, which indicates that the magnitudeof the strain can be correctly measured in this strain range.
Then, we performed a similar experiment using the POF. The
normalized power-change distributions measured when the
5-mm-long section was heated to 26 (room temperature),

Fig. 5. (Color online) (a) Normalized power-change distributions along a
POF measured at four temperatures. Each distribution was shifted by 1.
(b) Maximal power changes plotted as a function of temperature. The dottedline is a linear ﬁt.

35, 45, and 55 °C are shown in Fig. 5(a). The abrupt powerchanges were observed at the correct position along the POF.
The lengths of the power-changed sections moderately
agreed with the nominal resolution (0.21 m). The magnitudeof the maximal power change was proportional to thetemperature change as shown in Fig. 5(b), indicating thepotential feasibility of distributed temperature sensing.
Hereafter, we discuss the unique features of the beyondnominal-resolution eﬀect of SA-BOCDR in detail. The ﬁrst
discussion is regarding the measurement capability of thelocation, length, and magnitude of applied strain, when thestrain is shorter than the nominal spatial resolution. The strain
location can be derived from the system output as the centerof the nonzero-power-change section, and the strained length
can be given by subtracting the nominal spatial resolution
from the length of the nonzero-power-change section, as
already mentioned. The strain magnitude can also be givenby the maximal power change and the ratio of the strained
length to the nominal spatial resolution. Thus, if the SNR of
the system is suﬃciently high, these three parameters ofstrain can be theoretically obtained from the measured
power-change distribution, although the SNR may not be
suﬃciently high in most practical cases.
Next, we discuss the validity of claiming that the 2-mmlong strained SMF section and the 5-mm-long heated POF
section were detected. When a 2-mm-long strain was applied
to the SMF using epoxy glue, unintended strain distribution

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Jpn. J. Appl. Phys. 57, 020303 (2018)

H. Lee et al.

Fig. 6. Normalized power-change distribution along a silica SMF.
A 2-mm-long silica SMF with diﬀerent BFS was, as pseudostrain, spliced
between two silica SMFs.

was inevitably induced along the SMF inside the glue. This
may result in a practically longer strain to be measured. To
avoid this eﬀect and to prove that the 2-mm-long strain was
truly detected, we performed an additional experiment. By
splicing a 2-mm-long SMF with a diﬀerent BFS (11.02 GHz
at room temperature), we induced pseudostrain with a lengthof exactly 2 mm and measured the power-change distribution[the other experimental conditions were the same as thoseused in Fig. 4(a)]. The result shown in Fig. 6 clearly indicates that the 2-mm-long pseudostrain was truly detected. As
for the temperature measurement, it is diﬃcult to completely
avoid the inﬂuence of the thermal conduction, and thedetected heated section may be slightly longer than 5 mm;
however, the conclusion that this system has the capability todetect a 2-mm-long section with a diﬀerent BFS remains true.
The third discussion is regarding the limitation of thedetectable strained length. In this experiment, it was restricted
to 2 mm because of the deteriorated SNR. Therefore, the limitation could be enhanced by improving the SNR by using
higher-power incident light or by applying a larger strain. For
instance, in Fig. 4(a), a clear peak (which can be discriminatedfrom the noise ﬂoor even when the power-change becomeshalf) was observed at 0.2% strain; this may indicate that strain
with a length of less than half of 2 mm, namely of submillimeter length, can be potentially detected by SA-BOCDR.
Lastly, we discuss the possibility of employing this technique in other types of Brillouin sensors. We consider thatthe beyond-nominal-resolution eﬀect can also be given to
BOCDA systems, because they work on a similar principleof BOCDR (the ﬁnal system output of BOCDA can be expressed by the same mathematical forms as that of BOCDR).8)
As BOCDA exploits stimulated Brillouin scattering, which ismuch larger than spontaneous scattering, we predict that the
beyond-nominal-resolution eﬀect of SA-BOCDA will havehigher performance than SA-BOCDR and deserve developingin the future, despite its shortcomings in terms of the two-endaccess nature and the complexity of the setup.
In conclusion, ﬁrst, we experimentally characterized the“beyond-nominal-resolution” eﬀect of SA-BOCDR using thesilica SMF. When the nominal spatial resolution was constant, the measured maximal power change decreased almost
linearly with decreasing length of the strained section. Even
when the strained length was over 50 times shorter than thenominal spatial resolution, power change was still clearlyobserved at the correct position. Subsequently, by exploiting
this eﬀect, we demonstrated detection of the 2-mm-longstrained section in the silica SMF and a 5-mm-long heatedsection in the POF. In both cases, the power-change distributions at four diﬀerent strains and temperatures were measured, and the maximal power changes at each distribution

increased almost linearly with increasing strain and temperature. These results indicate that, even when the strained(or heated) section is much shorter than the nominal spatial
resolution, if the SNR is suﬃciently high, its length and themagnitude of the applied strain (or temperature change) can
be potentially calculated from the measured power-change
distribution. Thus, we anticipate that this work, which has
proven the capability of detecting extremely short strained=
heated sections along optical ﬁbers, will greatly enhance thepractical applicability of SA-BOCDR in the near future.
Acknowledgments This work was supported by JSPS KAKENHI Grant
Numbers 17H04930 and 17J07226, and by research grants from the Japan Gas
Association, the ESPEC Foundation for Global Environment Research andTechnology, the Association for Disaster Prevention Research, the Fujikura
Foundation, and the Japan Association for Chemical Innovation.

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020303-4© 2018 The Japan Society of Applied Physics